1. Field of the Invention
The present invention relates to a surface acoustic wave duplexer used in a wireless communication equipment, for example, a cellular phone, and more specifically, to a surface acoustic wave duplexer which suppresses a high frequency wave which occurs at a side that is higher than a pass band.
2. Description of the Related Art
In cellular phones, a surface acoustic wave duplexer is used to separate a transmission-side signal and a receiving-side signal. Here, the suppression of a twofold wave and a threefold wave of the transmission-side frequency is required.
In Japanese Unexamined Patent Application Publication No. 9-98046 (patent document 1), circuitry in which a low-pass filter is connected in a surface acoustic wave duplexer is disclosed in order to meet these demands. FIG. 20 is a diagram illustrating the circuitry of the surface acoustic wave duplexer described in the patent document 1. In the surface acoustic wave duplexer 201, a transmission-side surface acoustic wave filter 203 and a receiving-side surface acoustic wave filter 204 are connected to a common signal terminal 202 which is connected to an antenna. Also, a first low-pass filter 205 is connected between the common signal terminal 202 and the transmission-side surface acoustic wave filter 203, and a second low-pass filter 206 is connected between the common signal terminal 202 and the receiving-side surface acoustic wave filter 204.
The low-pass filters 205 and 206 have parallel capacitors C1 and C2 and an inductor L which is connected in series.
In addition to a method of using a low-pass filter which is described in patent document 1 described above, a technique is conventionally known in which a trap is formed by using an open stub and a short stub. Thereby, the attenuation of the twofold wave and the attenuation of the threefold wave of the transmission-side frequency are improved.
In Japanese Unexamined Patent Application Publication No. 11-68512 (patent document 2), an example of a method for forming a capacitance element on a piezoelectric substrate of a surface acoustic wave device is disclosed. FIG. 21 is a schematic plan view illustrating the surface acoustic wave device 211. In the surface acoustic wave device 211, surface acoustic wave filters 213 and 214 are disposed on a piezoelectric substrate. Also, a capacitance element 215 for matching impedance is similarly disposed on a piezoelectric substrate 212. The capacitance element 215 is composed of comb-shaped electrodes as shown in FIG. 21, and the direction in which electrode fingers of the comb-shaped electrodes are aligned is turned 90 degrees with respect to the propagation direction of the surface acoustic wave in the surface acoustic wave filters 213 and 214.
Also, in Japanese Unexamined Patent Application Publication No. 5-167388 (patent document 3), in a surface acoustic wave duplexer, an inductance L, which is formed of a metal strip line on a glass epoxy substrate or ceramic substrate, is connected between a surface acoustic wave filter having a relatively high frequency and an antenna-side common terminal. A structure is taught in which the inductance L is a phase-rotation element and operates to increase the impedance of the attenuation band of the low-frequency side of the surface acoustic wave filter of the side to which the inductance L is connected.
In the surface acoustic wave duplexer 201 described in patent document 1, low-pass filters 205 and 206, which are composed of parallel capacitors C1 and C2 and an inductor L connected in series, are connected to both of the transmission-side surface acoustic wave filter 203 and the receiving-side surface acoustic wave filter 204. Thereby, the attenuation of frequency higher than the pass band has been improved. Therefore, not only attenuation of a twofold wave and a threefold wave of the transmission-side frequency, but the attenuation of the high frequency side is improved. However, there has been a problem in that the insertion loss becomes large.
When forming a surface acoustic wave duplexer by a trap-type filter using the above-described open stub, short stub, or other suitable arrangement, by setting the trap positions to a frequency position of the twofold wave and the threefold wave of the transmission-side frequency, the attenuation of the above-described twofold wave and the threefold wave is improved. However, when forming a trap filter using an open stub, or a short stub, the area occupied by the trap filter within the package of the surface acoustic wave duplexer becomes large. Thus, miniaturization of the surface acoustic wave duplexer has been difficult.
In patent document 2, as described above, in a surface acoustic wave filter formed using a piezoelectric substrate, a structure has been disclosed in which a capacitance element is formed by arranging a comb-shaped electrode in a direction in which the alignment direction of electrode fingers is turned 90 degrees with respect to the surface acoustic wave propagation direction of the surface acoustic wave filter. However, the capacitance element 215 is used simply for a matching element of the surface acoustic wave filters 213 and 214.
Also, in patent document 3, the above-described inductor L is simply disclosed as a phase-rotation element in the surface acoustic wave duplexer.